Polyethylene and polypropylene are called commodity plastics. Polyethylene and polypropylene are very inexpensive, good in formability, and lightweight because they have a specific gravity of 1.0 or less. Therefore, the commodity plastics are widely used for housings such as in home electric appliances, information equipment including personal computers, and office automation equipment, interior automotive trim, and the like.
However, the commodity plastics have low mechanical strength, low toughness, and low stretchability. For these reasons, it is generally difficult to use the commodity plastics as alternative materials for metallic materials, for example.
A typical method for increasing the toughness and stretchability of such a commodity plastic is to blend a rubber material into the commodity plastic. This method, however, presents a problem in that it causes a significant reduction in mechanical strength. In addition, this method also presents a problem in that it has difficulties in blending a rubber-based material into a commodity plastic to disperse it well in the commodity plastic.
In the meantime, in order to improve the mechanical strength and other properties of a commodity plastic to use as an alternative material for a metallic material, it can be considered to orient molecules of the commodity plastic or increase the proportion of crystals.
Patent Literature 1 described below discloses, for the purpose of increasing the mechanical strength of a commodity plastic, a method for producing a polymer oriented crystalline body including: an oriented melt formation step of stretching a melt of polymer, such as polyethylene or polypropylene, at a strain rate not lower than a critical stretching strain rate to put the polymer melt into an oriented melt state; and a quenching crystallization step of cooling the polymer melt while maintaining it in an oriented melt state, thereby crystallizing it. In this case, in order to obtain a polymer melt, the polymer is heated at a temperature not lower than the melting point of the polymer. In this production method, the crystallinity of the polymer can be relatively high.